Ir Complexes With Mixed-Valent Bidentate Ligands: From Complex Synthesis to Catalytic Application.

The bidentate ligands Ph2P(E)NHPPh2 (E =  S, Se) feature phosphorous atoms in two different oxidation states and provide the potential to tune chemical reactivity through the choice of donor E. The mononuclear complexes [Ir(COD){κ2-P,E-Ph2P(E)NPPh2}] (E =  S 1a, Se 1b) were prepared from the dimer [IrClCOD]2 (COD =  1,5-cyclooctadiene) and the ligands Ph2P(E)NHPPh2 (E =  S, Se) in the presence of KOtBu. Reaction of complexes 1a and 1b with CO afforded complexes [Ir(CO)2{κ2-P,E-Ph2P(E)NPPh2}] (E =  S 2a, Se 2b). The carbonylation of 1a was studied using UV-vis absorption and Raman spectroscopies. A transient five-coordinate intermediate [Ir(CO)(COD){κ2-P,S-Ph2P(S)NPPh2}] 1a-I was observed. We support our findings with DFT calculations correlating the computed CO stretching frequencies with experimental values for this intermediate. The potential of complexes 1a, 1b, 2a, and 2b in the dehydrogenation of 2-naphthylmethanol is explored, with dehydrogenation yielding 2-naphthaldehyde and H2. This reactivity contrasts with the pathway reported earlier with other Ir catalysts, where additionally decarbonylation occurs. The yields with the COD derivatives, 1a and 1b, are moderate and higher than those of the carbonyl complexes, 2a and 2b. This difference highlights the effect of changing the ancillary ligands on the chemical behavior of the catalysts.